Fair Value Fair Value (Details 3) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013 MMBTU	Dec. 31, 2012 MMBTU
Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Weighted Average Forward Basis Price	0.10	0.03
Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Weighted Average Forward Basis Price	2	3
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(1.10)	(0.63)
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(8)	(2)
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	4.22	1.95
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	15	16
Fair Value, Measurements, Recurring [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 98	$ 147
Derivative Liabilities	(79)	(151)
Fair Value, Measurements, Recurring [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	9	40
Derivative Liabilities	(26)	(56)
Fair Value, Measurements, Recurring [Member] | Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	82	102
Derivative Liabilities	(51)	(93)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	127	160
Derivative Liabilities	(131)	(173)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	34	24
Derivative Liabilities	(48)	(62)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	88	134
Derivative Liabilities	$ (83)	$ (111)